A Macroevolutionary Perspective on Multiple Sexual Traits in the Phasianidae (Galliformes)

SAGE-Hindawi Access to Research International Journal of Evolutionary Biology Volume 2011, Article ID 423938, 16 pages doi:10.4061/2011/423938 Research Article A Macroevolutionary Perspective on Multiple Sexual Traits in the Phasianidae (Galliformes) RebeccaT.Kimball,ColetteM.St.Mary,andEdwardL.Braun Department of Biology, University of Florida, P.O. Box 118525, Gainesville, FL 32611, USA Correspondence should be addressed to Rebecca T. Kimball, [email protected]fl.edu Received 2 October 2010; Accepted 26 February 2011 Academic Editor: Rob Kulathinal Copyright © 2011 Rebecca T. Kimball et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Traits involved in sexual signaling are ubiquitous among animals. Although a single trait appears sufficient to convey information, many sexually dimorphic species exhibit multiple sexual signals, which may be costly to signalers and receivers. Given that one signal may be enough, there are many microevolutionary hypotheses to explain the evolution of multiple signals. Here we extend these hypotheses to a macroevolutionary scale and compare those predictions to the patterns of gains and losses of sexual dimorphism in pheasants and partridges. Among nine dimorphic characters, including six intersexual signals and three indicators of competitive ability, all exhibited both gains and losses of dimorphism within the group. Although theories of intersexual selection emphasize gain and elaboration, those six characters exhibited greater rates of loss than gain; in contrast, the competitive traits showed a slight bias towards gains. The available models, when examined in a macroevolutionary framework, did not yield unique predictions, making it difficult to distinguish among them. Even with this limitation, when the predictions of these alternative models were compared with the heterogeneous patterns of evolution of dimorphism in phasianids, it is clear that many different selective processes have been involved in the evolution of sexual signals in this group. 1. Introduction that taxa may have multiple sexually dimorphic traits as a result of the separate action of competition and mate The evolution of sexually dimorphic traits, such as arma- choice (e.g., [1]). If intrasexual and intersexual processes ments, bright coloration, or exaggerated traits has long been are acting independently, we would expect the evolution of of interest to evolutionary biologists, as such traits are often dimorphism in a trait classically associated with competition thought to reduce fitness through natural selection (e.g., to be unlinked (over evolutionary time and in a given taxa) [1]). However, in spite of the potential negative consequences with the evolution of dimorphism in a signaling trait that of these traits, many taxa exhibit not just one, but often might be associated with mate choice. multiple, sexually dimorphic traits. In nearly all systems Although the evolution of a sexually dimorphic trait examined, there is evidence that these multiple, sexually via competition and a second trait via mate choice may be dimorphic signals are used in both intrasexual (competition, expected within the same taxon, how can we explain the typically male-male competition) and intersexual (mate presence of multiple sexually dimorphic traits that are likely choice, typically female mate choice) interactions. Traits to have evolved through the same selective pressure? In the traditionally thought to have evolved via competition include case of competition, where each individual trait may confer ff those that directly a ect the physical competitive ability of a net competitive advantage (e.g., each type of armament males, such as larger body size, antlers, or other armaments, increases male competitive ability in an additive fashion), while traits typically associated with mate choice include it would be predicted that multiple traits should accumu- signals such as coloration or specialized ornaments such late over evolutionary time. In contrast, assessing multiple as modified feathers that are unlikely to have arisen via signals used for intersexual interactions is likely to be costly natural selection [1]. There is a long-standing expectation for female receivers, making it difficult to understand why 2 International Journal of Evolutionary Biology females would bear the cost of assessing multiple signaling competitive advantage (e.g., [14, 15]). Just as the assessment traits in mate choice. Thus, the majority of theories to explain of multiple signals is likely to be costly for females choosing multiple sexual signals has developed in the context of mate mates, the assessment of multiple signals indicating compet- choice (reviewed in [2, 3]). itive advantages in intrasexual interactions are also likely to Because signals can be costly to assess, produce, and be costly. Thus, we argue that many of the hypotheses put maintain, the development of theory to explain the gain forward to explain multiple cues in intersexual communica- and maintenance of multiple traits used in intersexual tion (mate choice) should also apply to the signals used in signaling has focused on the identification of plausible intrasexual interactions as well (e.g., [16]). mechanisms to explain why multiple signals, rather than a In extending the predictions of the existing theory single signal, are used by a receiver. Candolin [2]reviewed to the macroevolutionary scale, we made the simplifying seven classes of explanations including (1) different messages assumptions that signals of the same mode (e.g., coloration) are conveyed by the different signals, (2) that there are back- are more likely to be redundant in the information they up or redundant signals, (3) some signals are used primarily convey, whereas the presence of multiple signals of different in species recognition, (4) some signals are unreliable or modes (e.g., color patterns versus specialized structures) are Fisherian cues and thus not informative, (5) two signals more likely to communicate multiple different messages or are received more efficiently than one (receiver psychology), to act in concert. These assumptions can be logically argued (6) different sensory environments favor different signals, and are supported by data (e.g., [17, 18]butsee[19]). and (7) intersexual conflict/antagonistic coevolution results The upper portion of Table 1 focuses on the models in a dynamic in which older signals become uninformative that deal with signaling in general (applicable to both mate antes and newer signals arise. In his more recent review, choice and competition), summarizes the major classes of Bro-Jørgensen [3] also summarizes this theory, albeit slightly hypotheses to explain multiple sexual signals, and provides differently. In addition to the explanations discussed by our macroevolutionary expectations for sexual dimorphism Candolin [2], Bro-Jørgensen [3] expands the list to include and the pattern of trait accumulation and loss over time the rare male effect (i.e., the hypothesis that an unusual signal from each of models. In the lower portion of Table 1,we is always advantageous) and proposes a novel explanation discuss hypotheses that are specific just to mate choice. for the advantage of multiple sexual signals: the idea that Specifically, this includes rare male advantages in mate choice fluctuating signaling environments favor different signals at [9], antagonistic coevolution in sexual signaling [10, 11], and different times. Bro-Jørgenson [3] argues that since dynamic Fisher Runaway processes [4, 12, 13]. These models do not social and physical signaling environments are the rule, make specific predictions about the types of traits that should rather than the exception, this dynamic hypothesis may be a evolve but have been framed in a dynamic context and thus more general explanation for multiple signaling systems than make more explicit macroevolutionary predictions than the what has been proposed previously. majority of hypotheses. As can be seen, there is a wealth of plausible hypotheses To evaluate support for the macroevolutionary predic- available to explain the use of multiple sexual signals tions of the models, we used a comparative framework to (Table 1). A critical task at present is to evaluate whether or examine the distribution of sexual dimorphism in multiple not there is any dominant mechanism at work across taxa, traits in the family Phasianidae (chickens, pheasants, and or whether many different hypotheses will be necessary to allies). Although this is a very broad-brush approach, it has understand the ubiquity of multiple sexual signals. The exist- the potential to highlight general patterns and trends. Thus, ing hypotheses are framed in a strongly microevolutionary this can be viewed as the first step for more detailed studies context. As a result, we have detailed predictions about the in specific clades that further refine our understanding of the expression of multiple traits and their use and costs within evolution of multiple sexual traits. species (or populations) [2],butmuchlessclarityabout the expectations at the macroevolutionary (above species) scale regarding patterns of trait diversification, loss, or the 2. Methods association of traits in lineages where multiple sexual signals occur. However, looking across lineages over evolutionary 2.1. Study System. The avian family Phasianidae (within time should clarify whether

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